Chapter 12
File Management
Eighth Edition
By William Stallings
Operating
Systems:
Internals
and Design
Principles
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Chapter 12
File Management
Eighth Edition By William Stallings
Operating
Systems:
Internals
and Design
Principles
Files
Data collections created by users
The File System is one of the most important parts of the OS to a user
Desirable properties of files:
Long-term existence
- files are stored on disk or other secondary storage and do not disappear when a user logs off
Sharable between processes
- files have names and can have associated access permissions that permit controlled sharing
Structure
- files can be organized into hierarchical or more complex structure to reflect the relationships among files
File Structure
Four terms are
commonly used when
discussing files:
Field Record File Database
Structure Terms
Field
basic element of data
contains a single value
fixed or variable length
File
collection of related fields that can be treated as a unit by some application program
fixed or variable length
Record
Database
collection of similar records treated as a single entity may be referenced by name access control restrictions usually apply at the file level
collection of related data
relationships among elements of data are explicit
designed for use by a number of different applications
consists of one or more types of files
Minimal User Requirements
Each user:
1 •^ should be able to create, delete, read, write and modify files
2 • may have controlled access to other users’ files
3 •^ may control what type of accesses are allowed to the files
4 •^ should be able to restructure the files in a form appropriate to the problem
5 •^ should be able to move data between files
6 •^ should be able to back up and recover files in case of damage
7 •^ should be able to access his or her files by name rather than by numeric identifier
Indexed Pile Sequential Sequential
Logical I/O
Basic I/O Supervisor
Figure 12.1 File System Software Architecture
Basic File System
Disk Device Driver Tape Device Driver
Indexed Hashed
User Program
Basic File System
Also referred to as the physical I/O level
Primary interface with the environment outside the computer
system
Deals with blocks of data that are exchanged with disk or tape
systems
Concerned with the placement of blocks on the secondary storage
device
Concerned with buffering blocks in main memory
Considered part of the operating system
Basic I/O Supervisor
Responsible for all file I/O initiation and termination
Control structures that deal with device I/O, scheduling, and file
status are maintained
Selects the device on which I/O is to be performed
Concerned with scheduling disk and tape accesses to optimize
performance
I/O buffers are assigned and secondary memory is allocated at this
level
Part of the operating system
Access Method
Level of the file system closest to the user
Provides a standard interface between applications and the file
systems and devices that hold the data
Different access methods reflect different file structures and
different ways of accessing and processing the data
Directory management
Access method (^) Blocking Disk scheduling
File allocation
File Structure
Records
File management concerns
Figure 12.2 Elements of File Management
Operating system concerns
Physical blocks in main memory buffers
Physical blocks in secondary storage (disk)
User access control
User & program comands (^) Operation, File name
Free storage management
File manipulation functions
I/O
File Organization Types
Five of the common file organizations are:
The pile
The sequential file
The indexed indexed fileThe^ sequential file
The direct, or hashed, file
(a) Pile File
(c) Indexed Sequential File
(d) Indexed File
Figure 12.3 Common File Organizations
Variable-length recordsVariable set of fields Chronological order (b) Sequential File
Fixed-length recordsFixed set of fields in fixed order Sequential order based on key field
Main File
OverflowFile
Indexlevels
Exhaustiveindex Exhaustiveindex Partialindex
(variable-length records)Primary File
Index 12
n
The Sequential
File
Most common form of file
structure
A fixed format is used for
records
Key field uniquely identifies
the record
Typically used in batch
applications
Only organization that is
easily stored on tape as well
(a) Pile File as disk
ble-length records
ble set of fields
nological order
(b) Sequential File
Fixed-length records
Fixed set of fields in fixed order
Sequential order based on key field
Exhaustive Exhaustive Partial
Indexed
Sequential File
Adds an index to the file to support random access
Adds an overflow file
Greatly reduces the time required to access a single record
Multiple levels of indexing can be used to provide greater efficiency in access
(a) Pile File
(c) Indexed Sequential File
Main File
Overflow File
Index levels Index
1
2
n